In the field of optics, a combiner is an optical apparatus that combines two images together, from either the same side of the combiner (reflective/reflective), or transmissive/transmissive) or from the two different sides of the combiner (reflective/transmissive). Often times, optical combiners are used in heads up displays (“HUDs”), which allow a user to view a computer generated image (“CGI”) superimposed over an external view. The HUD enables the user to view the CGI without having to look away from her usual viewpoint. The term HUD originated from its use in avionics, which enabled a pilot to view information while looking forward with his head up, as opposed to looking down at cockpit instruments. Some conventional HUDs include holographic combiners and angled transparent substrates.
However, holographic combiners have a number of drawbacks. They are expensive to fabricate, difficult to mass produce, and have limited life spans (e.g., begin to degrade due to temperature, humidity, pressure and other harsh environmental conditions).
Angled transparent substrate combiners have been used in automobiles to present the driver with HUD information on the windshield. These optical combiners are made of a clear see-through substrate upon which an external image source displays the CGI. However, since the clear see-through substrate is typically a flat substrate without optical power so as not to distort the external FOV, the clear substrate must be angled (e.g. near 45 degrees) and bulky external magnification lenses are used to expand the CGI over the display region. The bulky external lenses and angled nature of the clear see-through substrate combiners do not lend themselves well to compact arrangements, such as head mounted displays (“HMDs”).